1. Field of the Invention:
The present invention relates o a gate turn-off semiconductor component in the form of a field-controlled thyristor. Such a component is described, for example, in EP-A2 0,178,387.
2. Discussion of the Background:
Field-controlled power semiconductor components are known with various structure and under various designations. Within this group of known components, a distinction is drawn essentially between two types of operation, namely unipolar structures such as, for example, field effect transistors of the MOSFET or JFET type which majority-carrier conduction and also components with bipolar carrier injection such as, for example, the field-controlled thyristor FCTh (Field Controlled Thyristor) or the static induction thyristor SITh (Static Induction Thyristor).
For physical reasons, the last named bipolar structures, whose operation is explained in the publication mentioned in the introduction, are particularly of interest for applications in the high-power range.
This mode of operation of the known structures is based as a rule on the application cf the JFET (Junction Field Effect Transistor) principle for the control: regions with charge-carrier depletion, which extend into the current-conducting channel region as the gate voltage increases, and finally interrupt or turn off the current flow by constricting the channel region, are produced in finely subdivided gate or control zones which alternate with cathode regions by applying an appropriate gate voltage and building up an expanding space charge region.
In the absence of a gate voltage, the field-controlled thyristor is in the conducting state (ON state) and is only turned off by applying a suitable gate voltage. For the dimensioning and design of such a thyristor structure this means that, in the conducting state, as low a conducting-state resistance (ON resistance) as possible is desirable in order to limit the drop in power across the component at maximum current flow. On the other hand, the gate-cathode structure of the component should be so designed that as good a controllability as possible is achieved, i.e. high powers can be switched with low gate voltages and gate currents.
On the basis of a structure of an FCTh known from EP-A1 0,121,068, in which the p-doped gate regions are disposed on the floors of the trenches, which separate the individual cathode fingers from each other, EP-A2 0,178,387 proposes to extend the p-doped gate regions to the trench walls in order thereby to achieve an improvement of the blocking gain and consequently of the controllability of the FCFh.
This extension of the gate regions to the trench walls achieves field control over the entire depth of the channel extending in the cathode finger. This ensures that the particular cathode finger is turned off with a sufficiently low gate voltage.
However, the improvement in the control properties achieved with the extended gate regions entails problems for the on-state behaviour of the thyristor: in the ON state the n-doped channel in the cathode finger is not flooded with charge carriers since the holes injected from the anode drain away via the p-doped gate regions in the trench walls. This chargecarrier depletion in the channel region results in a high ON resistance even though the gate electrode is not at a fixed potential ("floating gate").